The field of the invention is that of dual function circuit board substrates and the invention relates more particularly to improved dual function circuit board substrates and to circuits and circuit elements formed therewith.
A known dual function circuit board substrate has an ultrathin film, sheet or layer of a metal such as a nickel-chromium alloy or the like of relatively high electrically resistivity deposited on one side of a thin copper material or the like by sputtering or the like. A thin sheet or layer of an organic electrially insulating material is bonded to the other side of the sheet of high resistivity metal in any conventional way, and a heat-sink metal layer is adhered to the opposite side of the organic insulating layer material, preferably by using the organic layer material as a heat-bonding adhesive or the like, for withdrawing heat from the sheet of metal of high electrical resistivity. The thin sheets of metal of high electrical conductivity and resistivity are selectively etched or otherwise removed from the organic layer material to form circuit paths in a selected electrical circuit, and the metal of relatively high electrical conductivity is etched or otherwise removed from selected portions of that circuit to form one or more resistor elements in the circuit. By using such a dual function substrate, resistor elements of substantial resistance are adapted to be very compactly accommodated in a circuit with high circuit density by a circuit designer with each resistor element being securely and reliably connected in the circuit in a very convenient manner by bonding of the relatively high electrical conductivity metal to the high resistivity metal at each end of the resistor element.
In that arrangement, the thickness of the high resistivity metal is not variable in the circuit design by the user of the dual function substrate. However the high resistivity layer of the substrate is said to have selected sheet resistivity characteristics whose units are ohms per square, a square being a portion of the thin high resistivity material in which the width and length of the resistor element are equal. With that relationship, a resistor element formed in the circuit with a wide resistor portion and a long length can have the same effective resistance as a resistor element of smaller width if the smaller resistor is also of appropriately shorter length. Accordingly, it is usually desirable to form resistor elements which are proportioned to provide desired electrical resistance levels using the smallest possible area of the circuit to achieve compact circuit design. Frequently however it is found that heat build-up in the circuit is a limiting factor in achieving a desired circuit density, particularly heat build-up in the area of such resistor elements, so that some of the density improvement capabilities of the dual function type of substrate in incorporating resistor elements in a circuit is not fully utilized.